This invention relates generally to lifting jacks of the screw type, and more particularly relates to an improved tube sleeve member for such lifting jacks.
Screw jacks are well known to motorists and others as powerful and useful devices for lifting trucks and other vehicles. For a screw jack to be competitive in today's marketplace, the jack must be low in offered price, high in strength, and safe in operation. The component parts of the jack must offer efficient and effective operation, long service life, and must be adapted for high-quantity production at low per-item cost. A successful screw jack, its parts, and methods for parts production are discussed in the 1937 U.S. Pat. No. 2,096,050 to Lucker.
An important part of such a jack is a lifting sleeve, which can be considered to be a hollow tube provided, at least at one end, with an internal thread. An external key portion prevents tube rotation in the adjacent sleeve. As explained in the '050 patent, a one-piece lifting sleeve of this type can be formed by upsetting one end of a tube stock segment. In practice, hot upsetting equipment was used. Recently imposed government requirements have now begun to render the hot upsetting method of producing screw jack sleeve tubes and the end product sleeves themselves somewhat less commercially attractive.
Providing these sleeve tubes in production-run-sized quantities and at commercially attractive per-item costs is not an easy task. For example, counter-drilling a tube from one end so as to leave a short length of full thickness wall for subsequent tapping from one end requires a relatively long drilling time. Projection welding a key clip on the tube adds to production time and expense, and controlling the quality of the welded key-tube joint has proved difficult -- and, of course, adds further to production time and costs. Forming the necessary key by cold upset methods requires gathering a relatively large amount of metal from adjacent portions of the tube blank. When the metal is gathered and formed into the key, so little metal may remain in nearby portions of the tube blank that the internal tube thread cannot be properly formed.
Welding threadable bushings to tube blanks has proved expensive. Arc welding a bushing to a tube blank requires relatively complicated machining of the bushing and tube blank to adapt them for mechanized inert gas arc weldings. Quality assurance procedures must be imposed during the welding operation, and these procedures add to the finished item expense. Finally, the completed weld bead must be machined flush with the tube outer diameter and surface -- yet another item of expense.
Friction welding a threadable bushing to a tube sleeve also requires careful quality control procedures. After welding, the tube internal surface must be bored, and the outer surface must be turned to remove welding flash. Again, these steps add to production costs.
Other methods of production are experimental in nature, and for various reasons do not lend themselves readily to the production of large numbers of tube sleeve end products at relatively low cost.
It is therefore an object of the present invention to provide an unobjectionable method for producing high quality screw jack sleeve tubes at minimal per-item cost.
Another object is to provide a method of manufacturing screw jack sleeves without using expensive hot upsetting equipment. A related object is to provide an end product tube which is not subject to the possibility of various undesireable characteristics introduced by hot forming processes.
Yet another object is to provide a screw sleeve tube and method of manufacturing that sleeve tube which permits the use of end-product material, for example, bushing material, tailored to functional sleeve requirements. A related object is to provide such a sleeve tube in a format which is of minimal cost, yet which utilizes materials of maximal effectiveness.
Yet another object is to provide a sleeve tube for a screw jack or like device in which the shoulder under the threaded section will be sharp and consistent so as to permit the exit end of the tapped thread to be consistently formed for ease of sub-assembly on automated equipment.